Halogen-free flame retardant additive

ABSTRACT

The invention relates to a halogen-free flame retardant additive, essentially comprising hypophosphorous acid metallic salts coated inorganic hydrates and/or organic salts, useful as a flame retardant for polymer compositions, alone or in combination with other flame retardants and optionally further conventional components.

The present invention relates to a halogen-free flame retardant additive, essentially comprising hypophosphorous acid metallic salts coated with inorganic hydrates and/or organic salts, useful as a flame retardant for polymer compositions, alone or in combination with other flame retardants and optionally further conventional components.

BACKGROUND OF THE INVENTION

Halogen free flame retardant additives are of increasing interest in the thermoplastic polymers market. Basic requirements for these products are good processing in compounding and moulding conditions, good mechanical properties in the solid state and good flame retardancy in reinforced and unreinforced polymers.

Hypophosphorus acid metal salts, also called hypophosphites or inorganic phosphinates (phosphorus valence state=+1) have been reported as effective halogen free flame retardant additives for polymers. Thermoplastic polyester moulding materials containing a phosphinic acid salts have been described in the art, see for instance WO 03/014212 (equivalent to U.S. Pat. No. 71,692,812) and WO 99/57187 (equivalent to U.S. Pat. No. 6,503,969).

According to WO 03/014212, a polyester comprising a mixture of a phosphinic acid salt having a preferred particle size distribution and a nitrogen flame retardant, preferably mixed in advance and then feed to the melt, confer a particularly desired set of properties to the moulded composition.

According to WO 99/57187, a polyester comprising a mixture of a phosphinic acid salt and a nitrogen containing flame retardant, excluding melamine cyanurate, shows good flame retardant properties.

The phosphinic acid salts disclosed in the above documents apparently show a good flame retardant performance but at same time they cause the degradation of the polymeric composition to which they are added.

In fact, said additives chemically interact with the polymeric structure, provoking degradation or crosslinking, depending on the chemical nature of the specific polymer and of the additives. Crosslinking and degradation are of course unwanted phenomena in thermoplastic processing, as they cause a change in the rheology of the polymers and in the resultant mechanical properties of polymer moulding.

For instance, crosslinking of thermoplastic polymers results in an increased viscosity of the melt, with subsequent increasing in temperature if the melt is continuously subjected to shear forces. Temperature plays a fundamental role in accelerating this speed of reaction as well as in decomposing additives themselves. If crosslinking is high the thermoplastic is reverted into a so called thermoset polymer.

SUMMARY OF THE INVENTION

One object of the present invention is to provide halogen-free flame retardant additives showing very good processing behaviour and mechanical performances, thus overcoming the drawbacks of the prior art additives.

It was in fact surprisingly found out that applying a particular surface coating to hypophosphorous acid metallic salts results in more performing flame retardant additives for polymeric compositions.

DESCRIPTION OF THE INVENTION

So, according to one of its aspects, a subject-matter of the present invention is a hypophosphorous acid metallic salt characterized in that it is surface-coated with at least one compound selected from:

(a) alkali-metal or alkali-earth hydrates;

(b) hydrotalcite or hydrotalcite-like compounds; and

(c) alkali-metal or alkali-earth organic acid salts.

The hypophosphorous acid metallic salt of the invention is herein after also called “surface coated hypophosphorous acid metallic salt” or “surface-coated hypophosphite salt” and includes any metallic salt of hypophosphorous acid, such as any natural or synthetic alkali-metal and alkali-earth metal salts, for instance magnesium hypophosphite, calcium hypophosphite and aluminium hypophosphite. According to the present invention, the term “alkali-metal or alkali-earth hydrates” includes for instance magnesium hydroxide and aluminium hydroxide.

According to the present invention, the terms “hydrotalcite” and “hydrotalcite-like compounds” designate natural or synthetic compounds made of aluminium-magnesium-hydroxycarbonate, optionally hydrated, and derivatives thereof. Examples of hydrotalcite-like derivatives are compounds which are known also as layered double hydroxides or anionic clays, which chemical composition can be expressed by the general formula M^(II) _(1-x)M^(III) _(x)(OH)₂A^(n−) _(x/n)yH₂O where M^(II) and M^(III) are divalent and trivalent metal cations and A^(n−) is an n-valent anion, respectively. One example of commercially available synthetic hydrotalcite derivative is DHT-4A, provided by Kyowa.

According to the present invention, the term “alkali-metal or alkali-earth organic acid salts” means any natural or synthetic organic acid alkali-metal or alkali-earth salt. The term “organic acid” includes aromatic organic acids, such as benzoic acid, and aliphatic acids, such as fatty acids C8-C22, e.g. caprylic, capric, lauric, myristic, palmitic, stearic, arachidic, behenic or mixtures.

Preferred alkali-metal or alkali-earth organic acid salts are sodium stearate, magnesium stearate, calcium stearate, sodium benzoate and potassium benzoate. According to a preferred embodiment, preferred compounds (a) to (c) are magnesium stearate, sodium stearate and mixtures thereof.

According to a preferred embodiment, the hypophosphorous acid metallic salt is surface-coated by one or more compounds selected from magnesium hydroxide, synthetic hydrotalcite, sodium benzoate, potassium benzoate, sodium stearate and calcium stearate.

Also, different hypophosphorous acid metallic salts may be mixed and surface-coated by one or more compounds (a) to (c) above, to provide valuable flame retardant additives, according to the invention.

According to the present invention, the hypophosphorous acid metallic salt is surface-coated by intimate contacting of the hypophosphorous acid metallic salt and one or more compounds (a) to (c) above, preferably in a solvent, such as water, followed by filtering and drying of the product thus obtained.

Alternatively, the hypophosphorous acid metallic salt is surface-coated by mechanical grinding in a milling machine and optionally mixing the dry powders in a slow or high speed mechanical mixer.

The ratio hypophosphorous acid metallic salt/compounds (a) to (c) is preferably from 100/1 to 5/1 (w/w), preferably 100/1 to 10/1 (w/w).

The surface coating process of a hypophosphorous acid metallic salt with a compound selected from (a) to (c) above, represents a further aspect of the present invention.

If desired, a binding agent can be used in the surface-coating process of the invention to improve the adhesion of the coating compounds to the surface of the hypophosphorous acid metallic salt. Illustrative examples of binding agents are organic binders, such as synthetic or natural waxes, modified waxes, liquid hydrocarbons or epoxide resins.

A detailed description of the process of the invention is given the experimental part of the present application.

The surface-coated hypophosphorous acid metallic salts of the invention show improved flame-retardant performances with respect to the compounds known in the art. They can be used as flame retardant, alone or in combination with further conventional flame retardants or with processing aids, process and heat stabilizers, UV stabilizers, antidripping agents, pigments, mould release agents, nucleates, inorganic fillers, fibers, etc.

More particularly, they can be advantageously added to polymeric compositions and articles made thereof, such as thermoplastic polymeric compositions, polyesters or polyamides, either reinforced or unreinforced with glass fibers.

The polymeric compositions comprising the surface-coated hypophosphorous acid metallic salts of the invention show improved processing performances, that is to say, they may be moulded at high temperature and for a long time with a reduced changing of molecular weight of the polymer.

Also, the polymeric compositions comprising the surface-coated hypophosphorous acid metallic salts show good mechanical properties, as impact, tensile and flexural properties of the moulded polymer composition are only minimally affected from the addition of the novel flame retardant.

The flame retardant properties of surface-coated hypophosphorous acid metallic salts of the invention ranks V0 according to UL 94 and pass Glow wire test. Details of the flame retardant properties as well as comparative tests are reported in the experimental section.

For their use as flame retardant agents, the surface-coated hypophosphorous acid metallic salts of the invention may be added to the polymeric composition in a ratio polymeric composition/surface-coated hypophosphorous acid metallic salts which varies from 50/1 to 1/1 preferably from 20/1 to 3/1, for instance 5/1 to 4/1 (w/w).

The polymeric compositions comprising one or more surface-coated hypophosphorous acid metallic salts of the invention represents a further subject-matter of the invention.

The polymeric composition of the invention may comprise different surface-coated hypophosphorous acid metallic salts according to the invention.

According to a preferred embodiment, the polymeric compositions of the invention further comprise an epoxide resin and/or an organic binder.

According to another preferred embodiment, the polymeric compositions of the invention further comprise a nitrogen containing flame retardant.

According to another preferred embodiment, the polymeric compositions are PBT (polybutyleneterephthalate) resins.

The polymeric compositions of the invention are suitable to manufacture many different articles. Such articles represent another subject-matter of the invention.

The invention will be illustrate by means of the following examples in a non-limitative way.

In the following experimental part, the “Examples” relate to the surface-coated hypophosphorous acid metallic salts of the invention, whereas the “Comparative Examples” relate to the flame retardant compounds known in the art. In the same way, “Test Examples” designate the assays which have been carried out by using the designate surface-coated hypophosphorous acid metallic salts of the invention and “Comparative Test Examples” the assays which have been carried out by using the flame retardant compounds known in the art.

EXPERIMENTAL PART

In the examples the following components were used:

Commercially Available Polymers:

Polyamide 6,6 glass filled 30% (Latamid 6,6 GF30, by Lati)

Polybutyleneterephtalate (Niblan V100, by Soredi)

Polybutyleneterephtalate glass filled 30% (Niblan F30, by Soredi)

Lubricants: Pentaerythritolmonostearate (Loxiol P861, by Cognis), PTS Ethylen Bis Stearamide (EBS, by Croda) Stabilizers:

Hindered phenol heat stabilizer (Irganox 1098, Ciba)

Hypophosphites:

Aluminium hypophosphite (IP-A, by Italmatch Chemicals) Magnesium hypophosphite, anhydrous (IP-G, by Italmatch Chemicals) Calcium hypophosphite (IP-C, by Italmatch Chemicals)

FR Synergists:

Melamine cyanurate (Melagard MC25, by Italmatch Chemicals)

Surface Coating Agents:

Liquid epoxy resin modified to make it readily dispersable in water (Epikote 255, by Hexion) Toramide—curing agent for the epoxy resin Magnesium hydroxide (Magnifin H5 and H10 by Martinsweerk; Kisuma 5A by Kisuma Chemicals) Magnesium oxide (Sigma Aldrich), MgO Zinc oxide (Sigma Aldrich), ZnO Synthetic hydrotalcite (DHT-4A, Kyowa) Sodium benzoate (Velsicol), Na benzoate

Potassium Benzoate (ProBenz PG, by Velsicol), K Benzoate

Sodium stearate (Undesa), Na stearate

Zinc Borate (Borax) Zinc Sulfide (Sachtolith HDS by Sachtleben), ZnS

Calcium carbonate (Sigma Aldrich), CaCO3 Calcium stearate (Sogis), Ca stearate

Comparative Example 1

10 grams of Epikote 255 epoxy resin are dispersed with mechanical stirring into 30 cc of deionized water, and added to 200 grams of IP-A dispersed in 300 cc of deionized water. The emulsion is stirred for 15 minutes, then further 10 grams of Toramide emulsionized into 30 cc of deionized water are added and the final emulsion is left at 80° C. for 4 hours with mechanical stirring. The solid is filtered, washed and dried at 120° C. in an oven under vacuum, giving 209 grams of powder. Thermal Gravimetric Analysis of the powder measured with a SETARAM instrument model STA 92-16.18 in air at a scanning speed of 10° C. show a DTG peak at 330° C., practically identical to the pure IP-A in the same conditions (331° C.).

Example 2

10 grams of Epikote 255 epoxy resin are dispersed with mechanical stirring into 30 cc of deionized water, and added to 200 grams of IP-A and 10 grams of Magnifin H5 dispersed in 300 cc of deionized water. The emulsion is stirred for 15 minutes, then further 10 grams of Toramide emulsionized into 30 cc of deionized water are added and the final emulsion is left at 80° C. for 4 hours with mechanical stirring. The solid is filtered, washed and dried at 120° C. in an oven under vacuum, giving 214 grams of powder.

Example 3

In a laboratory ceramic ball mill jar model Giuliani are introduced 180 grams of IP-A and 5 grams of Magnifin H5. The jar is left working for 30 minutes and the powder discharged.

Thermal Gravimetric Analysis of the powder measured with a SETARAM instrument model STA 92-16.18 in air at a scanning speed of 10° C. show a DTG peak at 346° C., higher than pure IP-A in the same conditions (331° C.).

Examples 4-12, 20, 24-38 and Comparative Examples 13-19, 21-23

In the same laboratory mill than Example 3 and with the same procedure different surface coated hypophosphite are prepared using the following ingredients and quantities:

Comparative Example Example number number Hypophosphite Surface Coating 4 IP-A = 180 grams Magnifin H5 = 10 grams 5 IP-A = 180 grams Magnifin H5 = 20 grams 6 IP-A = 180 grams Magnifin H5 = 40 grams 7 IP-A = 180 grams DHT-4A = 20 grams 8 IP-A = 180 grams Na benzoate = 20 grams 9 IP-A = 180 grams K benzoate = 20 grams 10 IP-A = 180 grams Na stearate = 20 grams 11 IP-A = 180 grams Ca stearate = 5 grams 12 IP-A = 180 grams Ca stearate = 10 grams 13 IP-A = 180 grams Melagard MC25 = 40 grams 14 IP-A = 180 grams Melagard MC25 = 20 grams 15 IP-A = 180 grams Zn Borate = 40 grams 16 IP-A = 180 grams CaCO3 = 20 grams 17 IP-A = 180 grams CaCO3 = 40 grams 18 IP-A = 180 grams ZnS = 10 grams 19 IP-A = 180 grams ZnS = 40 grams 20 IP-M = 180 grams Magnifin H5 = 5 grams 21 IP-M = 180 grams Zn Borate = 40 grams 22 IP-M = 180 grams ZnO = 20 grams 23 IP-M = 180 grams MgO = 20 grams 24 IP-M = 68 grams; Magnifin H5 = 12 grams IP-A = 120 grams 25 IP-C = 180 grams Magnifin H5 = 5 grams 26 IP-C = 180 grams DHT-4A = 5 grams 27 IP-C = 180 grams Ca stearate = 5 grams 28 IP-C = 180 grams Ca stearate = 10 grams 29 IP-C = 240 grams Magnifin H5 = 20 grams 30 IP-G = 240 grams Magnifin H5 = 20 grams 31 IP-A = 240 grams Magnifin H5 = 20 grams 32 IP-A = 260 grams; Magnifin H5 = 10 grams IP-G = 30 grams 33 IP-A = 200 grams; Magnifin H5 = 10 grams IP-G = 90 grams 34 IP-A = 295 grams Magnifin H5 = 5 grams 35 IP-A = 240 grams Magnifin H5 = 20 grams 36 IP-A = 240 grams Magnifin H10 = 20 grams 37 IP-A = 250 grams Magnifin H10 = 10 grams 38 IP-A = 250 grams Magnifin H10 = 5 grams

Test Examples and Comparative Test Examples 38-69

In table 1, the results of melt stability in PA 6,6 glass filled 30% in a laboratory torque rheometer plasticizer model Brabender (50 grams chamber) are shown. Polymer and additives are introduced in the machine, and torque is recorded as a function of time at different temperatures. Crosslinking of polyamide lead to dramatic step increase of torque with burning of the sample itself.

In Column 1 is reported the hypophosphite type, in Columns 2 and 3 the quantities of additives and polymer used.

In Columns 5, 6, 7 time is reported in minutes, indicating the end of the test at different temperatures. The higher the time lap, the better the process stability.

Column 7 reports the Test Example numbers and Column 8 the Comparative Test Example numbers.

Example 39 shows that Polyamide 6,6 glass fiber reinforced is stable up to 310° C. without additives.

Test Example 40 shows that IP-A affect the melt stability of polyamides, at 300° C. the melt burn immediately with flames.

Comparative Ex. 41 show that the epoxide surface coating does not improve melt stability of aluminium hypophosphite when used alone (compared to Comp. Ex. 40), but it does improve the performances of magnesium hydroxide coating very likely allowing a better sticking to the hypophosphite surface (see Comparative Ex. 41, Ex. 42)

Test Examples 43-46 show the improvement of magnesium hydroxide coating on melt stability.

Test Examples 47-52 show the improvement of synthetic hydrotalcite, sodium and potassium benzoate, sodium and calcium stearate. Sodium stearate (Ex. 50) is particularly effective, so due to his double nature of effective surface coating and partially wax behaviour is useful in mixed coatings together with magnesium hydroxide, improving stickiness of the coating to the hypophosphite particles. Comparative Test Examples 53-59 show that melamine cyanurate, zinc borate, calcium carbonate and zinc sulphide are not effective.

Comparative Test Example 60 and Test Example 61 show that magnesium hydroxide is also effective in improving melt stability of magnesium hypophosphite, whereas Comparative Test Examples 62-64 show that zinc borate, zinc oxide and zinc sulfide are not.

Test Example 65 show that magnesium hydroxide coating is also effective when used on mixtures of aluminium and magnesium hypophosphite.

Test Examples 66-70 show that magnesium hydroxide, synthetic hydrotalcite and calcium stearate are effective in improving melt stability of calcium hypophosphite into PA 6,6.

TABLE 1 Column 1 Column 2 Column 3 Column 8 Hypophosphite Hypophosphite PA 6.6 Column 4 Column 5 Column 6 Column 7 Comp. type quantity GF 30% 290° C. 300° C. 310° C. Ex. N° Ex. N° — — 50 grams >15 min >15 min >15 min 39 IP-A 9 grams 41 grams 6 min B ND 40 Comp. Ex. 1 9.5 grams 40.5 grams 5 min ND ND 41 Ex. 2 10 grams 40 grams 10 min ND ND 42 Ex. 3 9.25 grams 40.75 grams 8 min ND ND 43 Ex. 4 9.5 grams 40.5 grams 8 min ND ND 44 Ex. 5 10 grams 40 grams >15 min 8 min ND 45 Ex. 6 11 grams 39 grams >15 min >15 min ND 46 Ex. 7 10 grams 40 grams ND 4 min ND 47 Ex. 8 10 grams 40 grams >15 min 11 min ND 48 Ex. 9 10 grams 40 grams >15 min 9 min ND 49 Ex. 10 10 grams 40 grams >15 >15 ND 50 Ex. 11 9.25 grams 40.75 grams 7 min ND ND 51 Ex. 12 9.5 grams 40.5 grams 8 min ND ND 52 Comp. Ex. 13 11 grams 39 grams 4 min ND ND 53 Comp. Ex. 14 10 grams 40 grams 4 min ND ND 54 Comp. Ex. 15 11 grams 39 grams 3 min ND ND 55 Comp. Ex. 16 10 grams 40 grams 4 min ND ND 56 Comp. Ex. 17 11 grams 39 grams 4 min ND ND 57 Comp. Ex. 18 9.5 grams 40.5 grams 5 min ND ND 58 Comp. Ex. 19 11 grams 39 grams 4 min ND ND 59 IP-M 9 grams 41 grams 9 min ND ND 60 Ex. 20 9.25 grams 40.75 grams >15 min ND ND 61 Comp. Ex. 21 11 grams 39 grams 8 min ND ND 62 Comp. Ex. 22 10 grams 40 grams 6 min ND ND 63 Comp. Ex. 23 10 grams 40 grams 10 min ND ND 64 Ex. 24 10 grams 40 grams 8 min 6 min ND 65 IP-C = 9 grams 9 grams 41 grams 3 min ND ND 66 Ex. 25 9.25 grams 40.75 grams >15 min >15 min    8 min 67 Ex. 26 9.25 grams 40.75 grams ND 8 min ND 68 Ex. 27 9.25 grams 40.75 grams 7 min ND ND 69 Ex. 28 9.5 grams 40.5 grams 11 min ND ND 70 Legend: B = Burn immediately ND = Not Determined

Test Examples and Comparative Test Examples 71-79

Components reported in table 2 are compounded in a 20 mm twin screw extruded with a temperature profile in the range 250-270° C. Polymer is dried 1 night in an oven at 120° C. before extruding. After drying a second time the compound in the same conditions pellets were injection moulded at different thickness, and 5 specimens were conditioned for 24 hours at 23° C. and 50% humidity. Flammability have been reported according to UL-94 procedure. When tests do not meet V0, V1 and V2 an NC classification has been given, when it was not possible to extrude or to inject samples an ND classification was attributed.

Comparative Test Examples 71-74 show the individual effectiveness of aluminium, calcium and magnesium hypophosphites. Aluminium hypophosphite is highly effective (Comp Ex. 72), but not enough stable in the melt. As a matter of fact, stopping the extruder or the injection moulding machine for some minutes caused burning and flame. Calcium hypophosphite was not possible to extrude at all (Comp. Test Example 73), when magnesium hypophosphite show satisfactory melt stability (Comp. Ex. 74).

Test Examples 76-78 show that magnesium hydroxide coated aluminium and magnesium hypophosphites result in satisfactory flame retardancy and improved melt stability compared to the uncoated ones.

TABLE 2 Comp. Comp. Comp. Comp. Comp. Ex. Ex. Ex. PA 6.6 GF 30% 71 72 73 74 75 76 77 78 Irganox 1098 0.2% 0.2% 0.2% 0.2% 0.2% 0.2% 0.2% 0.2% EBS 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% IP-A  18%   6% IP-C  18% IP-M  18%  14% Ex. 5  20% Ex. 6  22% Ex. 24  20% UL-94 3.2 mm NC V0 ND V0 V0 V0 V0 V0 UL-94 1.6 mm NC V0 ND NC V0 V0 V0 V0 UL-94 0.8 mm NC V0 ND NC NC V0 NC ND Brabender >15 6 2 10 6 >15 >15 8 melt stability at 290° C. (minutes) Brabender >15 B* B*  8 B*    8 >15 6 melt stability at 300° C. (minutes) *Burn, with flame ND = Not Determined NC = Not Classified

Test Examples and Comparative Test Examples 79-87

Components reported in table 3 are compounded in a 20 mm twin screw extruded with a temperature profile in the range 210-230° C. The polymer is dried overnight in an oven at 120° C. before extruding. After drying a second time the compound in the same conditions pellets were injection moulded at different thickness, and 5 specimens were conditioned for 24 hours at 23° C. and 50% humidity. Flammability have been reported according to UL-94 procedure. When tests do not meet V0, V1 and V2 an NC classification has been given, when it was not possible to extrude or to inject samples an ND classification was attributed.

Melt Flow Index (MFI) measurements were done at 250° C. with a 2.16 kg load and were recorded after compounding and after 3 and 5 extrusion passage in a twin screw extruder at 210-230° C. in the same conditions for all the compositions in the table. Differences in MFI between the value after 5 passage and after compounding are reported as Delta MFI and they represent the entity of degradation (the higher the number, the higher the degradation of the polymer).

Comparative Test Examples 79-82 show the effect of the uncoated hypophosphites in blend with a nitrogen flame retardant synergist. Flame retardant performances are satisfied, but the Delta MFI is in the best case 3 times higher than to comparative Test Example 79 (blank).

Comparative Test Example 83 and Test Example 84 show the positive effect of surface coating during grinding compared to the simple blending of powder in a extruder, both in terms of flame retardancy and polymer degradation.

Test Examples 85-87 show that it is possible to reduce degradation of the polymer and to maintain excellent flame retardant performances especially by using aluminium hypophosphite or mixtures of aluminium and magnesium hypophosphite coated according to the present invention.

TABLE 3 Comp. Comp. Comp. Comp. Comp. Ex. Ex. Ex. Ex. PBT GF 30% 79 80 81 82 83 84 85 86 87 PTS 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% IP-A  12%  12% IP-C  12% IP-M  12% Magnifin H5   1% Ex. 31  13% Ex. 32  15% Ex. 33  15% Ex. 34  15% Melagard MC25  12%  12%  12%  11%  11%   5%   5%   5% UL-94 3.2 mm NC V-0 V-0 V-0 V-0 V-0 V-0 V0 V0 UL-94 1.6 mm NC V-0 V-0 V-0 V-2 V-0 V-0 V0 V0 UL-94 0.8 mm NC V-2 NC NC V2 V2 V2 V2 V2 MFI 22 20    48 21 24 25 20 19 22 MFI after 24 36   160 26 30 25 22 21 24 3 extrusion MFI after 26 50 >230 33 38 32 25 23 28 5 extrusion Delta MFI  4 30 >200 12 14  7  5  4  6 (5 ext-0 ext) NC = Not Classified

Test Examples and Comparative Test Examples 88-92

Components reported in table 4 are compounded in a 20 mm twin screw extruded with a temperature profile in the range 210-230° C. Polymer is dried overnight in an oven at 120° C. before extruding. After drying a second time the compound in the same conditions pellets were injection moulded at different thickness, and 5 specimens were conditioned for 24 hours at 23° C. and 50% humidity. Flammability have been reported according to UL-94 procedure. When tests do not meet V0, V1 and V2 an NC classification has been given, when it was not possible to extrude or to inject samples an ND classification was attributed.

Melt Flow Index (MFI) measurements were done at 250° C. with a 2.16 kg load and were recorded after compounding and after 3 and 5 extrusion passage in a twin screw extruder at 210-230° C. in the same conditions for all the compositions in the table. Differences in MFI between the value after 5 passage and after compounding are reported as Delta MFI and they represent the entity of degradation (the higher the number, the higher the degradation of the polymer).

Mechanical properties were measured using an Instron 45045 instrument, in the same conditions for all recipes in the table and according to normatives ISO 527-1 and ISO 179-1.

High values of Charpy impact, Tensile Strength at break, Elongation at break and flexural modulus are highly preferred and are shown by Test Examples 89-92 when compared to Test Example 88.

TABLE 4 Comp. Ex. Ex. Ex. Ex. PBT unfilled 88 89 90 91 92 PTS 0.2% 0.2% 0.2% 0.2% 0.2% IP-A  18% Ex. 35  18% Ex. 36  18% Ex. 37  18% Ex. 38  18% Melagard MC-25   6%   6%   6%   6%   6% Charpy impact  2.05  2.02  2.40  2.17  2.09 23° C. (KJ/m2) T.S. @ break (Mpa) 45.6 49.1 48.9 49.7 49.2 E.B. @ break (%)  2.5  3.4 3   3.4  2.8 Module E (Mpa) 3441    3439    3410    3393    3453    UL-94 3.2 mm V0 V0 V0 V0 V0 UL-94 1.6 mm V2 V2 V2 V0 V0 UL-94 0.8 mm V2 V2 V2 V2 V2 MFR 250° C. 20.0 20.1 20.9 25.8 25.4 MFI 250° C. 26.8 20.3 25.4 27.2 29.7 after 3 extrusion MFI 250° C. 35.9 32.2 31.1 31.3 35.1 after 5 extrusion Delta MFI 15.9 12.1 10.3  5.5  9.7 (5 ext-0 ext) 

1. A hypophosphorous acid metallic salt characterized in that it is surface coated with at least one compound selected from: alkali-metal or alkali-earth hydrates; (a) hydrotalcite or hydrotalcite-like compounds; and (b) alkali-metal or alkali-earth organic acid salts.
 2. The hypophosphorous acid metallic salt according to claim 1, which is selected from hypophosphorous acid natural or synthetic, alkali-metal salts and alkali-earth metal salts.
 3. The hypophosphorous acid metallic salt according to claim 2, which is selected from magnesium hypophosphite, calcium hypophosphite and aluminium hypophosphite.
 4. The hypophosphorous acid metallic salt according to claim 1, characterized in that it is surface-coated by one or more compounds selected from magnesium hydroxide, synthetic hydrotalcite, sodium benzoate, potassium benzoate, sodium stearate, and calcium stearate.
 5. The hypophosphorous acid metallic salt according to claim 1, characterized in that ratio hypophosphorous acid metallic salt/compounds (a) to (c) is from 100/1 to 5/1 (w/w).
 6. A process for the preparation of the hypophosphorous acid metallic salt according to claim 1, characterized in that a hypophosphorous acid metallic salt is surface-coated by intimate contacting of the hypophosphorous acid metallic salt and one or more compounds (a) to (c) followed by filtering and drying of the product thus obtained.
 7. Process according to claim 6, which is carried out in a solvent.
 8. A process for the preparation of the hypophosphorous acid metallic salt according to claim 1, characterized in that a hypophosphorous acid metallic salt is surface-coated by mechanical grinding in a milling machine and optionally mixing in a slow or high speed mechanical mixer.
 9. Process according to claim 6, wherein a binding agent is added.
 10. The use of the hypophosphorous acid metallic salt according to claim 1, as flame retardant in thermoplastic polymeric compositions, polyesters or polyamides, either reinforced or unreinforced with glass fibers.
 11. Polymeric composition which comprises at least one hypophosphorous acid metallic salt according to claim
 1. 12. Polymeric composition according to claim 11, characterized in that it comprises aluminium hypophosphite or a mixture of aluminium and magnesium hypophosphite surface-coated.
 13. Polymeric composition according to claim 12, characterized in that the ratio polymeric composition/surface-coated hypophosphorous acid metallic salts which varies from 50/1 to 1/1 (w/w).
 14. Polymeric composition according to claim 13, characterized in that the ratio polymeric composition/surface-coated hypophosphorous acid metallic salts which varies from 20/1 to 3/1 (w/w).
 15. Polymeric composition according to claim 11, characterized in that it further comprises an epoxide resin and/or an organic binder.
 16. Polymeric composition according to claim 11, characterized in that it comprises more than one surface-coated hypophosphorous acid metallic salts.
 17. Polymeric composition according to claim 11, wherein the polymer is selected from polyesters and polyamides.
 18. Polymeric composition according to claim 11, wherein the polymer is polybutyleneterephthalate (PBT).
 19. Polymeric composition according to claim 11, which further comprises a nitrogen containing flame retardant.
 20. Flame retardant additive for polymeric compositions which comprises at least one surface-coated hypophosphorous acid metallic salts according to claim
 1. 21. Article made of a polymeric composition according to claim
 11. 